The life-shortening effect of ionizing radiation as an occupational hazard of exposed physicians has been indicated through studies of mortality rates of radiologists. Physicians from other specialties with a lower risk of exposure represent an optimum comparison population since both groups enjoy similar socio-economic and medical advantages which can prolong life. A previous study by Drs. Seltser and Sartwell utilizing such a population confirmed that radiologists had a lower life expectancy than other specialists. The preliminary data obtained during the first year of this grant has indicated that the excess risk for radiologists who entered the society through 1954 has persisted. The objectives and method which are proposed for the renewal period are: 1. To add to the original population the new members of the five societies and to compare their mortality experiencies. Purpose: To determine whether the newer safety measures have reduced the life-shortening and carcinogenic effects of occupational exposure to radiation. 2. To estimate the level and duration of radiation exposure through a questionnaire. Purpose: To correlate these estimates of radiation exposure to mortality.